MPEG video coding (as well as other forms of variable length video coding) is frequently used to compress video signals for transmission and/or storage with reduced transmission or storage requirements. For example, a 10-Mb per second MPEG video stream can often be reduced to a 4-Mb per second bitstream (or less) while retaining a high degree of image quality. In many instances, the degradation in image quality is practically imperceivable to the casual viewer at typical viewing distances.
MPEG coding utilizes a discrete cosine transforming process in the coding process. A frame of video is divided into blocks with each block being represented in part by a block of coefficients for a discrete cosine transform. The block of coefficients spans from low frequency coefficients to high frequency coefficients as the block is traversed from the upper left-hand corner to the lower right-hand corner of the block, as commonly depicted. In order to compress the block, it is common for high frequency coefficients in the block to be dropped altogether since the human eye is least able to perceive the changes in the video signal represented by the high frequency coefficients. The more of these high frequency coefficients that are dropped, the greater the reduction in the bit rate, and thus the reduction in storage requirements or transmission requirements for the coded MPEG bitstream.
The process of dropping the higher frequency discrete cosine transform (DCT) coefficients is referred to as coefficient dropping. Coefficient dropping is used to effect bit rate reduction by reducing the high frequency content. Because these DCT coefficients have an inherent error by virtue of this form of bit rate reduction, a phenomenon known as drift can occur. Drift results because the reference frame used to form MPEG P and B frames in decoding is different from that used in the encoding process. This phenomenon of drift ultimately results in degraded image quality in the coded video signal. It is therefore desirable to minimize the amount of drift that occurs.